The instant invention relates to fiber lasers, and more particularly to a side-pumped fiber laser.
Rare-earth doped fiber lasers are typically considered to be low power devices because efficient longitudinal coupling of pump laser light into the small fiber core is difficult, and there are limitations to the actual fiber itself. Limitations for higher output powers are given by the available power of the applied laser diode arrays that can be focused onto the fiber when longitudinally pumping, and the damage threshold of the core with respect to absorbed pump light intensity. It is thus difficult to increase the output power by only longitudinal pumping. It has been suggested by some that the solution for this problem is the repetitive pump-light in-coupling from the side of the fiber, hence the term xe2x80x9cside-pumpedxe2x80x9d. For example, side pumping arrangements of lasers are disclosed in U.S. Pat. Nos. 5,455,838 and 4,794,615. Side pumping of fiber lasers has been described in the articles by M. Hofer et al, xe2x80x9cHigh-Power Side-Pumped Passively Mode-Locked Er-YB Fiber Laserxe2x80x9d IEEE Photonics Technology Letters, Vol. 10, No. 9, September 1998, and Weber, et al , xe2x80x9cSide-Pumped Fiber Laserxe2x80x9d, Applied Physics B, Lasers and Optics, Vol. 63, Pages 131-134 (1996). In the Hofer article, side pumping of a double clad fiber is accomplished by cutting a 90 degree V-shaped groove into the outer cladding of the fiber. A lens was used to focus diode emission onto the V-groove facet. In the Weber article, side pumping of a double clad fiber was accomplished by removing the silicone outer cladding on the upper side of the fiber over a length of about 1 mm. A prism with the same refractive index as the inner cladding is placed onto the stripped location. An index matching oil is applied between the prism and the fiber.
While the prior art attempts to side pump light into the fiber have shown that side coupling is possible, there is still a need in the industry to improve coupling efficiency and to improve configurations for manufacturing such side pumping arrangements.
In this regard, the instant invention provides a simple and efficient means for side-pumping light into the core of a single clad fiber. More specifically, the side-pumped fiber laser includes an optical fiber having a core and a cladding. The core has an index of refraction n1 and the cladding has an index of refraction n2, wherein n1 is greater than n2. The fiber further includes a coupling window disposed in a window channel formed in an upper side of fiber cladding. The fiber laser further includes a laser light source, such as a laser diode bar emitting a laser light beam, or multiple laser light beams that are directed through the coupling window wherein laser light is directly coupled into the core of the fiber.
The coupling window consists of an optical material having an index of refraction n3, wherein n3 is greater than n1. The window channel is formed by physically removing a portion of the cladding material. The optical material utilized to form the coupling material is deposited onto the fiber within the channel and becomes part of the integral structure of the fiber. The coupling window effectively forms an integrated prism to guide light into the fiber and enhance coupling of the light into the core. The optical material that forms the coupling window can be a graded index material or a step index material. The coupling window can be shaped as a rectangle, triangle or other geometric shape depending on the laser diode smile, i.e. emission pattern, and aperture to further enhance coupling efficiency. Since the window material is deposited directly onto the fiber, rather than glued onto the surface, there is no air gap and no need to use index matching gels, and thus the coupling efficient is increased and the whole structure is better integrated.
A second embodiment of the fiber laser includes a fiber wherein Bragg gratings are written into the core beneath the coupling window. The fiber further includes a reflective window (mirror) disposed in a second window channel formed in the opposing lower side of the cladding beneath the Bragg gratings. The reflective window consists of a reflective optical material deposited into the window channel. The reflective window can be used as a filter to filter out unwanted wavelengths. The reflective window can also be used to tune the wavelength of the fiber laser by using a feedback mechanism.
Accordingly, among the objects of the instant invention are: the provision of a side-pumped fiber laser having a high coupling efficiency; the provision of a side-pumped fiber laser that allows pumping of light directly into the core of the fiber through a window prism directly integrated into the structure of the fiber; the provision of a side-pumped fiber laser that can filter out unwanted wavelengths; the provision of a side-pumped fiber laser that is tunable; the provision of a side-pumped fiber laser including Bragg gratings to enhance and direct coupling of the light into the core; and the provision of a side-pumped fiber laser including repetitive side-coupling of light into the fiber.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.